The leafminer Tuta absoluta (Meyrick) is a major pest of the tomato crop and its development rate is known to decline when nitrogen availability for crop growth is limited. Because N limitation reduces plant primary metabolism but enhances secondary metabolism, one can infer that the slow larval development arises from lower leaf nutritive value and/or higher plant defence. As an attempt to study the first alternative, we examined the tomato-T. absoluta interaction in terms of resource supply by leaves and intake by larvae. Tomato plants were raised under controlled conditions on N-sufficient vs. N-limited complete nutrient solutions. Plants were kept healthy or artificially inoculated with larvae for seven days. Serial harvests were taken and the N, C, dry mass and water contents were determined in roots, stems and leaves. Leaf and mine areas were also measured and the N, C, dry mass and water surface densities were calculated in order to characterize the diet of the larvae. The infestation of a specific leaf lessened its local biomass by 8-26%, but this effect was undetectable at the whole plant scale. Infestation markedly increased resource density per unit leaf area (water, dry mass, C and N) suggesting that the insect induced changes in leaf composition. Nitrogen limitation lessened whole plant growth (by 50%) and infested leaflet growth (by 32-44%). It produced opposite effects on specific resource density per unit area, increasing that of dry mass and C while decreasing water and N. These changes were ineffective on insect mining activity, but slowed down larval development. Under N limitation, T. absoluta consumed less water and N but more dry mass and C. The resulting consequences were a 50-70% increase of C:N stoichiometry in their diet and the doubling of faeces excretion. The observed limitation of larval development is therefore consistent with a trophic explanation caused by low N and/or water intakes.
Over recent years, major progress in experimental approaches have bring insights about the ecological functions of volatile organic compounds (VOCs) released by plants. However, deciphering volatile emissions in a methodologically robust and ecologically relevant manner remains a challenging issue. A surge in interest is required to characterize potential blind spots in volatile sampling that could result in dramatic bias in our understanding of VOCs. In parallel, ecologists need to account for various environmental factors in order to address appropriately the sources of variations of VOCs. Here we use two common porous polymers, polydimethylsiloxane (PDMS) and Porapak Q, to collect VOCs released by tomato exposed to herbivory in combination with nitrogen shortage. We dissect two key features of volatile blends, i.e., their composition and their diversity. Upon nitrogen limitation, Porapak Q stresses the up-regulation of a common defensive compound (methyl salicylate), while herbivory induces three terpenes involved in the recruitment of natural enemies of Tuta absoluta (2-carene, ɑ-pinene and β-phellandrene). This study suggests that the combination of resource availability and herbivory governs the differential production of generalist and specific VOCs that are active against a broad spectrum or particular herbivore species, respectively. But PDMS was found unsuitable to observe such patterns in the composition of VOC emissions. Additionally, Porapak Q was found more sensitive than PDMS to track the increase in the diversity of stress-related VOC emissions upon nitrogen limitation. This suggests that plants growing with poor resources release more information in surroundings. We discuss particular implications for tri-trophic-mediated plant defences.
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